The Oceans F335

Just some revision cards outlining the key points in The Oceans. :)

  • Created by: Adele
  • Created on: 23-05-14 09:40

Entropy Changes

Entropy: Entropy is the measure of the number of ways of arranging molecules and distributing their quanta of energy.

The entropy change for a chemical system is the difference between the entropies of the products and the reactants - 

ΔSsys =  ΔS(produts) -  ΔS(reactants)

The entropy change for the surroundings depends on the tranfer of heat to/from the surroundings -

ΔSsurr = - ΔH/T  Where  ΔH must always be converted to joules.

ΔStotal =  ΔSsys +  ΔSsurr If  ΔStotal is positive then the reaction will occur spontaneously and if is equals zero, the reaction is at equilibrium.

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Energy Changes and Solutions

Lattice Formation Enthalpy ΔHLe = The energy change when 1 mole of a solid is formed from its separate gaseous ions.

Enthalpy Change of Hydration ΔHhyd = The energy change when an aqueous solution is formed from 1 mole of gaseous ions.

Enthalpy Change of Solution ΔHsolution = The enthalpy change when 1 mole of a solute dissolves to form a dilute solution. ΔHsolution = ΔHhyd(cation) +  ΔHhyd(anion) - ΔHLe

If ΔHsolution is negative or slightly positive, the solid will usually dissolve as the entropy change will be favourable and energy can be used from the surroundings. If the value is highly positive however, the solid won't dissolve because too much energy will be required.

Lattice and hydration enthalpies become more negative when:

  • The ionic charges increase
  • The ionic radii decrease
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Enthalpy Level Diagrams


The value for ΔHsolution is negative, meaning the solid will be soluble in the solvent used.

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Enthalpy Level Diagrams


The value for ΔHsolution in this enthalpy level diagram is slightly positive, meaning the solid could dissolve in the solvent. Any values higher than this, and the solid would not be able to dissolve as too much energy would be required.

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